In a typical inkjet recording or printing system, ink droplets are ejected from a nozzle at high speed towards a recording element or medium to produce an image on the medium. The ink droplets, or recording liquid, generally comprise a recording agent, such as a dye or pigment, and a large amount of solvent. The solvent, or carrier liquid, typically is made up of an aqueous mixture, for example, comprising water and one or more organic materials such as a monohydric alcohol, or a polyhydric alcohol.
An inkjet recording media typically comprises a support having on at least one surface thereof at least one ink-receiving layer (IRL). There are generally two types of IRLs. The first type of IRL comprises a non-porous coating of a polymer with a high capacity for swelling, which non-porous coating absorbs ink by molecular diffusion. Cationic or anionic substances may be added to the coating to serve as a dye fixing agent or mordant for a cationic or anionic dye. Typically, the support is a smooth resin-coated paper and the ink-receiving layer is optically transparent and very smooth, leading to a very high gloss “photo-grade” inkjet recording media. However, this type of IRL usually tends to absorb the ink slowly and, consequently, the imaged receiver or print is not instantaneously dry to the touch.
The second type of ink-receiving layer or IRL comprises a porous coating of inorganic, polymeric, or organic-inorganic composite particles, a polymeric binder, and optional additives such as dye-fixing agents or mordants. These particles can vary in chemical composition, size, shape, and intra-particle porosity. In this case, the printing liquid is absorbed into the open interconnected pores of the IRL, substantially by capillary action, to obtain a print that is instantaneously dry to the touch. Typically the total interconnected inter-particle pore volume of porous media, which may include one or more layers, is more than sufficient to hold all the applied ink forming the image.
As the desire for photographs reproduced by inkjet printing technology grows, there is increased demand for improved image quality. Historically, receivers with swellable layers of bydrophilic polymers on glossy resin-coated papers were used for photographs, but these receivers dried slowly and were inconvenient to handle until dry. Porous-design photo papers provide prints that are dry-to-the-touch upon exit from the printer. In addition, the demand for high color density requires a receiver with high capacity for ink. Lack of capacity results in pooling of ink droplets on the surface of the receiver, leading to the phenomena observed as coalescence or mottle. A further demand is for high-speed printing. Consequently, as ink flux increases capacity alone may not be sufficient for proper absorption of ink droplets.
In providing at least a partial solution to these demands, various technologies for fixing or immobilizing the ink droplets on the receiver surface have been proposed. This serves to reduce mixing that results in coalescence and increases the concentration of colorant at or near the surface, increasing density. In the case of dye-based aqueous inks used in inkjet printing, the dyes generally comprise anionic moieties and are known to complex with suitable cationic species, thus binding the dye near the surface to ensure high color density. For dye-based inks, the preferred fixing agent is often called a mordant and may comprise a salt of a quaternary nitrogen moiety, frequently in polymeric form, or a salt of a multivalent metal cation.
A particular challenge with pigment-based inks is that the penetration of the fluid portion of the ink may be slowed if the pigment particles partially block the pores of the media. Since the fluid stays on the surface longer, drops may mix and initiate coalescence and the appearance of mottle. The level of mottle can be significantly reduced by the addition of fixing agents. The preferred fixing agents are multivalent metal cations. One solution is to provide a salt of a cationic fixing agent in the receiver as manufactured and another is for the printer to deliver a solution to the receiver comprising such a salt either by coating, spraying or jetting. The solution may be applied to the receiver in various sequences, including immediately prior to, concurrently with, or immediately following jetting of the ink droplets.
Katsuragi, et al., in U.S. Pat. No. 6,550,903, disclose liquid compositions, ink sets, apparatus, and processes for inkjet recording on plain paper. A first liquid containing a polyvalent salt of a metal cation and a second liquid containing a coloring material are used in combination and applied on a plain paper so as to come into contact with each other. Katsuragi, et al., disclose the salt of a polyvalent metal cation with a polyhydroxycarboxylic acid for improving the waterfastness of pigment-based inks printed on plain paper. Furthermore, an improvement in image sharpness and a reduction in feathering are alleged, along with resistance to bleeding when different colors are printed adjacent to one another, specifically when one of the inks is a black ink. Printing systems that include printer-applied fixing solutions involve extra complexity, extra solution supplies and extra delivery systems. Drying times are increased when extra aqueous solutions are applied to the receiver. A problem of principal concern when jetting a fixing agent via printhead is that the fixing agent will contaminate the printhead and cause fouling and other concerns.
A problem not mentioned in '903, since it dealt only with a system employing plain, uncoated paper as the receiver, is that for glossy photo-quality media, the addition of salts of multivalent metal cations results in a severe loss of gloss in prints with pigment-based inks. Thus, a simple printing system is needed to provide photographs that are instantly dry-to-the-touch, employ colorants resistant to fade over a lifetime, and exhibit superb image quality with minimal coalescence and mottle, and high gloss.